A = {i:0 for i in "abcdefghijklm"}
s = input()
for i in s:
    if i in A:
        A[i] += 1
if max(A.values()) > 1 and list(A.values()).count(0)<2:
    for i,j in A.items():
        if j == 0:
            print(i)
elif max(A.values()) == 1 and list(A.values()).count(0)<2:
    for i in sorted(A):
        print(i)
else:
    print("Impossible")